The present invention relates to utilization of solar energy and, more particularly, to a solar absorber-reflector for alternately transmitting and repelling back solar heat resulting from incident solar radiation.
There already exist solar absorber-reflectors, such as the Venetian blinds taught in U.S. Pat. No. 4,002,159. These blinds, usually installed at room windows, have slats that are rotatable through 180 degrees, where one face is bearing a layer of material with a usual black nonselective light-absorbing surface, while the other face is a light-reflecting surface.
For room heating in cold weather, the venetian blind slats are oriented in such a way that their light-reflecting surfaces face the interior of the room, and their light-absorbing surfaces are directed outward. This is a solar-heating mode, in which the light-absorbing surface absorbs solar radiation, which is converted into heat and transferred into the room interior. In hot weather, the venetian blind slats are rotated through 180 degrees. This is a light-reflection mode, in which the solar radiation is reflected, preventing from overheating the room interior.
However, the thermal efficiency of such venetian blinds is rather low. First of all, high thermal emittance of the nonselective light-absorbing surface results in a loss of a significant part of the heat absorbed in solar-heating mode, due to the re-emission from the light-absorbing surface (unless the venetian blind is shielded by a window pane, and the warmed up air circulates into the room interior). Furthermore, when operating in the light-reflection mode, a certain heating of the light-reflecting surface occurs under exposure to solar radiation, resulting in an undesired heat transfer into the room interior.
The thermal efficiency of devices similar to the venetian blinds described above can be increased to a certain extent, by replacing the nonselective light-absorbing surfaces with selective solar radiation absorbing surfaces. Selective solar radiation absorbing surfaces are surfaces with reduced amount of re-emission losses of the absorbed solar radiation, and hence increased efficiency of solar heating. This is achieved by low thermal emittance combined with high absorptance of the solar radiation. Materials that can be used for manufacturing selective solar radiation absorbing surfaces exist in many varieties. (Further details can be found in the publication "Coating for enhanced photothermal energy collection" by C. M. Lampert, "Solar Energy Materials", 1, 319-341, 1979).
A method for making a multiple layer solar absorber-reflector is taught in U.S. Pat. No. 4,320,155. A transparent substrate such as soda-lime-silica glass is vacuum coated with a first layer of a reflective metal such as aluminum, a second layer of a light absorbing semi-conducting material such as germanium or silicon, and a third layer of an antireflective material such as silicon monoxide, for enhancing the absorption of the light absorbing layer.
However, the thermal efficiency of solar absorber-reflectors still remains rather low, even with the use of selective solar radiation absorbing surfaces. In the solar-heating mode, intensive heat transfer from the layer of material with the selective solar radiation absorbing surface to the room interior, is prevented due to poor thermal emittance of the light-reflecting surface. This results in overheating of the layer of material with the selective solar radiation absorbing surface, and hence in an increase of heat losses by both convection heat transfer and re-emission of additional heat by this surface. Furthermore, solar absorber-reflectors with a selective solar radiation absorbing surface, while operating in the light-reflection mode, still feature a certain heating of the light-reflecting surface under exposure to solar radiation, resulting in an undesired heat transfer into the room interior, as with the venetian blinds described above.
There exists a certain type of absorbers of solar radiation with thermal efficiency which is significantly higher than that of the above mentioned solar absorber-reflectors, when operating in the solar-heating mode. Such absorbers of solar radiation contain, besides the material layer with a selective solar radiation absorbing surface, an additional material layer with a heat-emitting surface facing the direction opposite to the selective solar radiation absorbing surface.
The above mentioned overheating of the material layer with the selective solar radiation absorbing surface does not occur in absorbers of solar radiation with a heat-emitting surface, due to intensive heat transfer from the layer of material with the selective solar absorbing surface into the room interior through the heat-emitting surface which increases the thermal efficiency of these absorbers of solar radiation. However, solar absorber-reflectors, utilizing a selective solar radiation absorbing surface together with a heat-emitting surface for the solar-heating mode, as well as a light-reflecting surface for the light-reflection mode, have not yet been practically configured. In order to obtain a surface most simple from the technological point of view, and possessing simultaneously the properties of light reflection and heat emittance, special paints can be applied. (Further details can be found in the publication "Estimation of the end-of-life optical properties of Z-93 thermal control coating for the space station Freedom", by M. M. Hasegava and H. W. Babel, AIAA Papers, AIAA-92-2168-CP, 1992, p. 88). However, these paints are very expensive, and require thorough and complicated pre-painting preparation of the surface. Therefore, solar absorber-reflectors utilizing these paints are not worthwhile economically. Furthermore, even such solar absorber-reflectors, while operating in the light-reflection mode, still feature a certain heating of the light-reflecting surface under exposure to solar radiation, resulting in an undesired heat transfer into the room interior, as with the previously mentioned solar absorber-reflectors.
There is thus a widely recognized need for, and it would be highly advantageous to have, a multi-layer solar absorber-reflector, that is simple to manufacture, install and operate, utilizing the solar energy for room heating in efficient manner, on an attractive economical basis. Namely, the properties of the solar absorber-reflector should include providing intensive heat transfer through a light-absorbing surface into the room interior in the solar-heating mode for heat accumulation, such as in cold weather, and reducing heating of a light-reflecting surface under exposure to solar radiation in the light-reflection mode, significantly preventing solar heat from penetrating into the room interior, such as in hot weather.